Control of supplied pressure in assisted ventilation

ABSTRACT

Methods and apparatus for detecting the occurrence of a potential or actual overpressure during assisted ventilation are described. A blower supplies 
     pressurized gas to a conduit, and in turn to a patient mask for connection with the entrance of a patient&#39;s airways. A pressure sensor detects the delivered pressure in the mask, which is provided to a controller. The controller has operation over the blower by way of a servo and motor. The controller determines a relatively longterm average of the pressure signal, and compares it against a threshold value. If the threshold value is approached or exceeded, the controller controls the blower and thus the supplied pressure to the patient. The effect of the control can be to limit or reduce the supplied gas pressure. The relatively longterm average can be of the order of minutes, or taken over ten or more breaths.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/981,243 filed Nov. 4, 2004 now U.S. Pat. No. 8,397,722 which is acontinuation of U.S. application Ser. No. 09/936,854 filed Jan. 2, 2002,now U.S. Pat. No. 6,840,240, which was a National Stage Entry of PCTapplication number PCT/AU00/00411 filed May 5, 2000, entitled “Controlof Supplied Pressure in Assisted Ventilation”, which claims the prioritydate of Australian Patent Application PQ0198 filed May 16, 1999, thedisclosures of which are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to Non Invasive Positive Pressure Ventilation(NIPPV) treatment apparatus for the provision of assisted ventilation.Particularly, the invention concerns the control of treatment pressuresupplied to a subject.

BACKGROUND ART

NIPPV apparatus function to supply a patient with a supply of cleanbreathable gas (usually air, with or without supplemental oxygen) at atherapeutic pressure or pressures, at appropriate times during thesubject's breathing cycle. The therapeutic pressure is also known as theventilation pressure.

NIPPV apparatus typically include a flow generator, an air filter, amask, an air delivery conduit connecting the flow generator to the mask,various sensors and a microprocessor-based controller. The flowgenerator may include a servo-controlled motor and an impeller. The flowgenerator may also include a valve capable of discharging air toatmosphere as a means for altering the pressure delivered to the patientas an alternative to motor speed control. The sensors measure, amongstother things, motor speed, gas volumetric flowrate and outlet pressure.The apparatus may optionally include a humidifier in the air deliverycircuit. The controller may include data storage capacity with orwithout integrated data retrieval and display functions.

In this specification, NIPPV apparatus will be referred to as “assistedventilation devices” which, in the broadest form, need not include allof the component features mentioned above.

Assisted ventilation devices are used for the treatment of manyconditions, for example respiratory insufficiency or failure due tolung, neuromuscular or musculoskeletal disease and diseases ofrespiratory control.

Common to all forms of assisted ventilation is the need to control thepressure being applied to the patient. It is a known prior art techniqueto detect the peak pressure and compare it against a maximum thresholdvalue. If the threshold value is exceeded an alarm state occurs, andcorrective action may be taken. This corrective action can be ashort-term reduction in supplied pressure, followed by an increase backto the previous pressure.

DISCLOSURE OF THE INVENTION

The present invention is directed to providing an alternative,advantageous approach to the problem of overpressure.

The invention discloses a method for controlling operation of anassisted ventilation device supplying pressurised gas to a patient, themethod comprising the steps of: determining a relatively longtermaverage of pressure of gas supplied to said patient; and controlling thepressure supplied by said ventilation device with regard to saidlongterm average.

The invention further discloses a method for detecting the occurrence ofa potential or actual overpressure during assisted ventilation,comprising the steps of determining a relatively longterm average ofventilation pressure, and determining whether the average approaches orexceeds a threshold value as being indicative of a potential or actualoverpressure occurring.

The invention further discloses a method for controlling operation of anassisted ventilation device supplying pressurised gas to a patient, themethod comprising the steps of: measuring the currently deliveredpressure; determining a relatively longterm average of the measuredpressure; comparing said average against a threshold value; and if thethreshold value is approached or exceeded, controlling the pressuresupplied by the device.

The invention yet further discloses assisted ventilation apparatus fordetecting a potential or actual overpressure condition, comprising: ablower to supply pressurised gas to a conduit, and in turn to a patientmask for connection with the entrance to a patient's airways: a pressuresensor to detect the delivered pressure of gas in the conduit or at themask, and provide a signal thereof; and a controller receiving saidpressure signal and having control over operation of the blower andoperable to determine a relatively longterm average of the pressuresignal and to control the supplied pressure with regard to said longtermaverage.

The invention yet further discloses assisted ventilation apparatus fordetecting a potential or actual overpressure condition, comprising: ablower to supply pressurised gas to a conduit, and in turn to a patientmask for connection with the entrance to a patient's airways; a pressuresensor to detect the delivered pressure of gas in the conduit or at themask, and provide a signal thereof; and a controller, receiving thepressure signal and having control over operation of the blower, andoperable to determine a relatively longterm average of the pressuresignal, compare the average against a threshold value, and if thethreshold value is approached or exceeded, to control the blower andthus the supplied pressure.

In one preferred form, an alarm state exists when said threshold isapproached or exceeded, and on the occurrence of an alarm state, theassisted ventilation apparatus issues an alarm. Additionally oralternatively, the blower can be controlled to be switched-off or to beplaced in a low pressure standby mode (for example 4 cmH₂O).

The invention further discloses a method for controlling operation of anassisted ventilation device supplying pressurised gas to a patient, themethod comprising the steps of: determining a relatively longtermaverage of supplied pressure; and controlling said supplied pressure asa function of a waveform template, a target patient ventilation and saidlongterm average.

In relation to control of supplied pressure, the blower can becontrolled to limit or reduce the supplied pressure. The reduction canbe a non-linear function of time and/or pressure. Particularly, thedegree of control can be stronger/greater as the threshold value isapproached.

The longterm average can, in one form, be of the order of minutes.

Alternatively, the average can be over ten or more breaths.

The threshold can be required to be exceeded for a minimum period oftime before the alarm state is assessed as occurring.

The invention is advantageous in that it approaches the problem ofoverpressure from a relatively longer time scale than in the prior art.This is considered to be a more appropriate approach to the medicalconditions that attend overpressure in assisted ventilation. Forexample, sustained overpressure causes a decrease in cardiac output,which would go largely untreated by the prior art arrangement discussedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a representative assistedventilation device, in the form of NIPPV apparatus;

FIG. 2 is a schematic block diagram of an overpressure detectioncircuit;

FIG. 3 shows traces of treatment pressure with time and the operation ofan embodiment of the invention;

FIG. 4 shows further traces of treatment pressure with time and theoperation of an embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS AND BEST MODE

An assisted ventilation device embodying one form of the invention isshown in FIG. 1, in which a blower comprising a motor 20 and an impeller10, supplies breathable gas to a mask 11 for communication with asubject's airway via a delivery tube or conduit 12 and exhausting toatmosphere via an exhaust 13. Airflow at the mask 11 is measured using apneumotachograph 14 and a differential pressure transducer 15. The maskflow signal from the transducer 15 is sampled by a microprocessor 16.Mask pressure is measured at a port 17 using a pressure transducer 18.The pressure signal from the transducer 15 is also sampled by themicroprocessor 16. The microprocessor 16 sends an instantaneous maskpressure request signal to a servo 19, which compares the pressurerequest signal with the actual pressure signal from the transducer 18 tocontrol a motor 20 driving the impeller 10. The microprocessor'ssettings can be adjusted via a serial port 21.

It is to be understood that the mask could equally be replaced with atracheotomy tube, endotracheal tube, nasal pillows, or other means ofmaking a sealed connection between the air delivery means and thesubject's airway.

In general terms, the invention is concerned with determining arelatively longterm average of ventilation pressure and avoidingoccurrence of overpressure with regard thereto.

In one embodiment, the microprocessor 16 determines the long-termaverage of the actual treatment pressure, P, and compares this against athreshold or maximum value, P _(max), If the threshold value is exceededthen corrective action may be taken.

The corrective action can be to issue an alarm, to switch-off theassisted ventilation device, to reduce the treatment pressure, or tocontrol the blower in a more complex manner, an example of which isdescribed in more detail below.

As shown in FIG. 2, the circuitry 30 receives a signal from the pressuretransducer 15 indicative of the pressure in the air delivery conduit ortube 12. The signal is amplified by an operational amplifier 32, thenlow-pass filtered 34 with a time constant of approximately one minute.Longer or shorter time constants would be appropriate depending on howlong it was considered safe for the subject to be exposed to arelatively high mean pressure. In one embodiment, the time constant canbe varied by way of an operator accessible control. The low-passfiltered signal passes to a comparator 36 where it is compared with areference pressure signal corresponding to 15 cmH₂O, representing P_(max). The output from the comparator passes to both the servo 19 and aresettable monostable/one-shot 38. The resettable monostable/one-shot 38is set to 30 seconds. Longer or short time periods would be suitable forspecific assisted ventilation applications.

If the output from the comparator 36 is ‘true’, an indication that thelow-pass filter signal exceeds P _(max), a “reduce” pressure signal issent to the servo 19 (shown in FIG. 1) on line 42. At this point, theresettable monostable/one-shot 38 starts to count down. If the countdown reaches zero, then a stop signal is sent to the servo 19 on line44. The count determines an adjustable tolerance on how long the alarmstate has occurred before corrective action is taken. If the output fromthe comparator 36 is “false”, there is no alarm state, and theresettable monostable/one-shot 38 is reset.

In another embodiment, implemented in software, the avoidance ofoverpressure is approached as the continuous monitoring of pressure as afunction of the longterm average of the pressure. Referring once againto FIG. 1, the microprocessor 16 receives a signal representing maskpressure from the transducer 18. The microprocessor 16 controls theservo 19 such that the desired treatment pressure achieved satisfies thefollowing equation:P=P _(o) +k·A·f(v,t)where:P is the pressure setting for the blower (degree of support) cmH₂O;P_(o) is a constant, the baseline pressure, chosen, for example, to keepthe upper airway open, or to balance intrinsic PEEP cmH₂O.

In one form,k=1.  [2a]

In other forms,k=k′, low pass filtered with time constant of 5 seconds  [2b]where:

$\begin{matrix}{k = \left\{ \begin{matrix}{0,} & {\overset{\_}{p} \geq {15\mspace{14mu}{cm}\mspace{14mu} H_{2}O}} \\{0.1,} & {\overset{\_}{p} = {14.9\mspace{14mu}{cm}\mspace{14mu} H_{2}O}} \\{1,} & {\overset{\_}{p} \leq {14.5\mspace{14mu}{cm}\mspace{14mu} H_{2}O}}\end{matrix} \right.} & \lbrack 3\rbrack\end{matrix}$and linearly in between.

The purpose of making k nonlinear on p is to provide strong control as P_(max) is approached, with less effect further away from P _(max). Thepurpose of low pass filtering is to reduce distortion of thewithin-breath pressure-time profile.

The pressure modulation amplitude, A (cmH₂O) is given by:A=g∫(V _(e) −V _(TGT))dt  [4]where g is a constant, V_(e) is the minute ventilation, and V_(TGT) isthe target ventilation.A may be truncated to lie between A_(max) and A_(min).f is a function of at least one of time, t, and respiratory airflow, vchosen to produce the desired pressure waveform. A range of functions isknown to those skilled in the art. One example function corresponding toa spontaneous mode bi-level ventilator is:

$\begin{matrix}{{f\left( {v,t} \right)} = \left\{ \begin{matrix}{1,} & {v > 0} \\{0,} & {otherwise}\end{matrix} \right.} & \left\lbrack {5a} \right\rbrack\end{matrix}$Another example function is:

$\begin{matrix}{{f\left( {v,t} \right)} = \left\{ \begin{matrix}{1,} & {t^{\prime} < T_{i}} \\{0,} & {otherwise}\end{matrix} \right.} & \left\lbrack {5b} \right\rbrack\end{matrix}$where:

t′=t modulo T_(not)

T_(i), =duration of inspiration

T_(not)=duration of a breath

This corresponds to a “timed-mode” bilevel ventilator, with

P=P_(o) during expiration; and

P=P_(o)+A during inspiration

A number of simulations have been performed to demonstrate an embodimentof the invention in practise.

In FIG. 3, there is a sustained rise in peak pressure. In the top traceis shown the effect without the use of the present invention. The meanpressure exceeds a chosen P _(max) of 15 cmH₂O, which is undesirable. Inthe second trace, this is corrected by an embodiment of the invention,where the mean pressure is kept close to P _(max). The bottom traceshows the factor “k”, and how it decreases from unity to approximately0.5.

In FIG. 4, there is a transient rise in peak pressure. In the top trace,even without the practise of invention, the mean pressure closelyapproaches but does not exceed P _(max), which is permissible, eventhough the instantaneous pressure goes very high. With the inventionpractised, (second trace), the resultant pressure is little affected,because k remains close to unity (bottom trace).

In this embodiment, as the mean pressure threshold is approached, thedegree of assistance is gradually reduced or limited. In anotherembodiment, both the baseline pressure, P_(o), and amplitude ofventilatory support, A, are progressively reduced as the mean pressure,p, approaches the desired threshold pressure, P _(max). The inventionhas been described with reference to a number of non-limiting examples,and it will be appreciated that the invention can be embodied innumerous forms.

The invention claimed is:
 1. A method for detecting the occurrence of apotential or actual overpressure during assisted ventilation, comprisingthe steps of: determining a relatively longterm average of ventilationpressure; and determining whether the average approaches or exceeds athreshold value as being indicative of a potential or actualoverpressure occurring.
 2. A method for controlling operation of anassisted ventilation device supplying pressurised gas to a patient, themethod comprising the steps of: measuring the currently deliveredpressure; determining a relatively longterm average of the measuredpressure; comparing said average against a threshold value; and if thethreshold value is approached or exceeded, controlling the pressuresupplied by the device.
 3. Assisted ventilation apparatus for detectinga potential or actual overpressure condition, comprising: a blower tosupply pressurised gas to a conduit, and in turn to a patient mask forconnection with the entrance to a patient's airways; a pressure sensorto detect the delivered pressure of gas in the conduit or at the mask,and provide a signal thereof; and a controller, receiving the pressuresignal and having control over operation of the blower, and operable todetermine a relatively longterm average of the pressure signal, comparethe average against a threshold value, and if the threshold value isapproached or exceeded, to control the blower and thus the suppliedpressure wherein, for the case of said longterm average exceeding saidthreshold, the controller operates subject to the condition that thetime in excess must be greater than a minimum period of time before itis determined that a potential or actual overpressure is occurring. 4.Apparatus as claimed in claim 3, wherein said controller controls thesupplied pressure by limitation or reduction.
 5. Apparatus as claimed inclaim 4, wherein, for the case of reducing supplied pressure, thecontroller reduces the pressure as a non-linear function of time and/orpressure.
 6. Apparatus as claimed in claim 3, further comprising meansfor indicating that an alarm state exists if the threshold value isapproached or exceeded.
 7. Apparatus as claimed in claim 3, wherein saidcontroller determines the longterm average in the order of minutes. 8.Apparatus as claimed in claim 3, wherein said controller determines thelongterm average over ten or more breaths.